Method and system for providing error compensation to a signal using feedback control

ABSTRACT

A method for providing error compensation to a signal includes providing error adjustment to a signal at a transmitter and communicating the signal over a channel at a channel speed. The method includes receiving the signal from the channel at the channel speed and sampling the signal at a speed less than the channel speed to yield a sampled signal. The method includes determining an error associated with the signal and determining compensation information for providing the error adjustment. The compensation information is based on the error and is determined at a compensation speed. The compensation speed is less than the channel speed. The method also includes communicating information to the transmitter for providing error adjustment. The information communicated to the transmitter may comprise the compensation information. The information communicated to the transmitter may also comprise intermediate information from which the compensation information is determined.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.10/359,361, entitled “METHOD AND SYSTEM FOR PROCESSING A SAMPLEDSIGNAL,” and U.S. patent application Ser. No. 10/359,379, entitled“METHOD AND SYSTEM FOR SIGNAL PROCESSING USING VECTOR OUTPUT FROM SCALARDATA,”both filed concurrently with the present application.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of data communication andmore specifically to a method and system for providing errorcompensation to a signal using feedback control.

BACKGROUND OF THE INVENTION

Signals received at a receiver are typically processed to compensate forinterference due to frequency dependent loss. As an example, adaptiveequalization is performed to compensate for inter-symbol interferencedue to channel loss. Known techniques for adaptive equalization havebeen used in the area of digital magnetic/optical recording and lowspeed digital communication such as modem and wireless communication.These techniques typically provide full speed signal processing at thespeed of the channel symbol rate for fast tracking error compensation atthe receiver for channel variation.

SUMMARY OF THE INVENTION

The present invention provides a method and system for providing errorcompensation to a signal using feedback control that substantiallyeliminates or reduces at least some of the disadvantages and problemsassociated with previous error compensation methods and systems.

In accordance with a particular embodiment of the present invention, amethod for providing error compensation to a signal includes providingerror adjustment to a signal at a transmitter and communicating thesignal over a channel at a channel speed. The method includes receivingthe signal from the channel at the channel speed and sampling the signalat a speed less than the channel speed to yield a sampled signal. Themethod includes determining an error associated with the signal anddetermining compensation information for the error adjustment. Thecompensation information is based on the error and is determined at acompensation speed. The compensation speed is less than the channelspeed. The method also includes communicating information to thetransmitter for providing error adjustment.

The information communicated to the transmitter may comprise thecompensation information. The information communicated to thetransmitter may also comprise intermediate information from which thecompensation information is determined. The intermediate information maycomprise the error associated with the signal. The method may alsoinclude accumulating a plurality of cross-correlated data points of thereceived signal cross-correlated with the error and forming across-correlation vector from the cross-correlated data points at thecompensation speed. The compensation speed may be less than the speed atwhich the signal is sampled and may be based on the cross-correlationvector. The method may also include sampling the determined error at asampling speed less than the channel speed.

In accordance with another embodiment, a system for providing errorcompensation to a signal includes an adjustable filter positioned beforea channel. The adjustable filter is operable to provide error adjustmentto a signal. The system includes a transmitter operable to communicatethe signal over the channel at a channel speed and a receiver operableto receive the signal from the channel at the channel speed. The systemincludes a sampling switch operable to sample the signal at a speed lessthan the channel speed to yield a sampled signal and an error calculatoroperable to determine an error associated with the signal. The systemalso includes adaptation control operable to determine compensationinformation for the error adjustment. The compensation information isbased on the error determined by the error calculator, and theadaptation control is operable to determine the compensation informationat a compensation speed. The compensation speed less than the channelspeed. The system also includes feedback control operable to communicateinformation to the adjustable filter for providing error adjustment.

Technical advantages of particular embodiments of the present inventioninclude an adaptive equalization system with feedback control thatdetermines error compensation instructions or control parameters from areceived signal and communicates such instructions to an adjustablefilter positioned before the channel over which a signal is to becommunicated. Thus, a transmitter with the adjustable filter is able toprovide pre-emphasis to a signal to compensate for expected distortions.Therefore, the efficiency of a network system utilizing the adaptiveequalization system may be improved.

Other technical advantages will be readily apparent to one skilled inthe art from the following figures, descriptions and claims. Moreover,while specific advantages have been enumerated above, variousembodiments may include all, some or none of the enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of particular embodiments of theinvention and their advantages, reference is now made to the followingdescriptions, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 illustrates a network system having a receiver with an adaptiveequalization system, in accordance with an embodiment of the presentinvention;

FIG. 2 illustrates an adaptive equalization system for processing areceived signal, in accordance with an embodiment of the presentinvention;

FIG. 3 illustrates an adaptive equalization system with subsamplingswitches, in accordance with a particular embodiment of the presentinvention;

FIG. 4 illustrates an adaptive equalization system utilizing anaccumulator for producing vector output, in accordance with a particularembodiment of the present invention;

FIG. 5 illustrates an adaptive equalization system utilizing anaccumulator for producing vector output, in accordance with anotherembodiment of the present invention;

FIG. 6 illustrates a network system utilizing adaptive equalization withfeedback control, in accordance with a particular embodiment of thepresent invention;

FIG. 7 illustrates an adaptive equalization system utilizing anaccumulator and feedback control, in accordance with another embodimentof the present invention; and

FIG. 8 is a flow chart illustrating a method for providing errorcompensation to a signal using feedback control, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a network system 10 having a receiver 20 with anadaptive equalization system 22, in accordance with an embodiment of thepresent invention. Adaptive equalization system 22 performs signalprocessing on a signal received at receiver 20. The signal processingmay be performed at a lower speed than the full channel speed of thesignal. Thus, the negative impact of signal processing on speed of thesignal may be reduced, and the signal speed through the channel maytherefore be improved.

Network system 10 includes network elements 12 and 14 coupled togetherby a channel 16. Network elements 12 and 14 may comprise servers,storage systems, routers, computer systems or any combination thereof.Channel 16 communicates signals between network elements 12 and 14.Channel 16 may comprise a cable having a length in the range of ten toone hundred meters, for example approximately twenty to forty meters.The speed of a signal travelling over channel 16 may be in the range ofmulti-gigabits per second, for example, approximately three gigabits persecond. As an example, channel 16 may operate according to 10 GigabitAttachment Unit Interface (XAUI) standards, which require a fixedfrequency of approximately one to five gigahertz, for example, 3.125gigahertz, and which are used for 10 Gigabit Ethernet communications.

Signals traveling at high speeds are susceptible to distortion resultingfrom a spread impulse response of channel 16. Channel 16 may causefrequency dependent distortion due to skin effect and dieletric loss.Such distortion may cause inter-symbol interference (ISI). To overcomeISI, multi-gigabit rate electrical communication over several tens ofmeters may utilize high-degree equalization over approximately twenty toforty dB. Particular embodiments of the present invention use advancedsignal-processing technologies to provide high-degree equalization tomulti-gigabit rate communications over channel 16.

Network element 12 includes a transmitter 18, and, as illustrated,network element 14 includes receiver 20. Transmitter 18 transmits a datasequence or signal over channel 16 to receiver 20. Adaptive equalizationsystem 22 provides high-degree equalization to overcome ISI in thesignal communicated through channel 16.

FIG. 2 illustrates an adaptive equalization system 30 for processing areceived signal, in accordance with an embodiment of the presentinvention. Adaptive equalization system 30 includes an adjustable filter32, a detector 34, a shift register 36, an error calculator 38 andadaptation control 39.

Adjustable filter 32 receives a signal that has been transmitted over achannel, such as channel 16 of FIG. 1. Adjustable filter 32 compensatesfor distortion in the signal caused by the channel. Control andadjustment of adjustable filter 32 is provided by adaptation control 39.Particular embodiments may utilize an adjustable filter implemented as adiscrete-time digital filter, a discrete-time analog filter or acontinuous-time analog filter.

After the signal has passed through adjustable filter 32, it travels todetector 34, which recovers the originally transmitted data sequence.Detector 34 may be any of a number of types of detectors, such as adecision feedback detector or a direct detector. Shift register 36provides a delay element to form a vector composed of a series of datapoints from the signal. The signal is sampled at a particular intervalbefore entering error calculator 38. Error calculator 38 performs signalprocessing to calculate an error associated with the sampled signal.Such error may comprise an amplitude error. The calculated error is thenused by adaptation control 39 to determine parameters or instructionsfor adjustable filter 32. Adjustable filter 32 uses such instructions toprovide compensation to subsequent signals for distortion caused by thechannel.

In the illustrated embodiment, component box 33 includes adjustablefilter 32, detector 34 and shift register 36; and component box 35includes error calculator 38 and adaptation control 39. Component boxes33 and 35 may be implemented as digital circuits, analog circuits or acombination of digital and analog circuits. The components of componentbox 33 operate at the speed of the channel, for example the speed ofchannel 16 of FIG. 1. The signal is sampled at a particular intervalbefore entering component box 35, so the components of component box 35operate at such interval. Since the components of component box 35 onlyoperate at a particular interval, such components operate at a sloweroperational speed than the components of component box 33. Thus, thenegative impact of the error calculation and adaptation controlfunctions of component box 35 on the speed of adaptive equalizationsystem 30 is reduced since those functions only operate at the intervalsof the sampled signal and not at the full channel speed of thecomponents of component box 33. Since error is only determined at theintervals of the sampled signal, continuous error calculation and fasttracking compensation is sacrificed. However, such fast trackingcapability is less important in multi-gigabit rate electricalcommunication for some applications. Therefore, the sampling of receivedsignals for signal processing reduces the negative impact of the signalprocessing on the full channel speed. Thus, a higher channel speed maybe obtained.

FIG. 3 illustrates an adaptive equalization system 40 with subsamplingswitches 52, in accordance with a particular embodiment of the presentinvention. Adaptive equalization system 40 receives a transmitted datasequence that has passed through channel 42. Adaptive equalizationsystem 40 includes an adjustable filter 44, a sampling switch 46, adetector 48, a shift register 50, subsampling switches 52, targetresponse filter 54, summation node 55, multiplication node 57,adaptation matrix 56, and integrator bank 58.

Adjustable filter 44 receives transmitted data sequence a_(k) after thesignal has traveled over channel 42. Channel 42 causes some distortionin the signal. Adjustable filter 44 compensates for such distortioncaused by the channel. Control and adjustment of adjustable filter 44 isdetermined by a compensation vector, filter coefficient vector c, whichis produced by integrator bank 58. Particular embodiments may include anadjustable filter located after a sampling switch or before a channelthrough which the signal is communicated, as illustrated with respect toFIGS. 5 and 7, respectively.

After the data sequence a_(k) has passed through adjustable filter 44,it is sampled at sampling switch 46. Sampling switch 46 yields a sampledsignal {tilde over (d)}_(k). The data sequence may be sampled atsampling time t=kT, where T represents a sampling period. Sampled signal{tilde over (d)}_(k) enters detector 48, which removes noise in thesignal to recover the originally transmitted noiseless data sequenceâ_(k).

Shift register 50 provides a delay element to form a vector composed ofa series of data points. The output of shift register 50 is a noiselessdata vector {circumflex over (a)}_(k). The length of the data vector{circumflex over (a)}_(k) produced by shift register 50 depends on thecharacteristics of the channel through which the signal was transmitted.

The output of shift register 50, data vector {circumflex over (a)}_(k),is sampled at subsampling switch 52 a at sampling time k=iN, where Nrepresents a subsampling rate, to yield data vector {circumflex over(a)}_(i). The subscript i corresponds to the index of the sub-sampleddata series, and {circumflex over (a)}_(i) is an abbreviated notation of{circumflex over (a)}_(iN) that is a sub-sampled data series withsub-sampling rate of N. Thus, {circumflex over (a)}_(i) refers to{circumflex over (a)}_(0N), {circumflex over (a)}_(1N), {circumflex over(a)}_(2N), {circumflex over (a)}_(3N), . . . {circumflex over (a)}_(iN),. . . . It should be understood that subsampling or subsampling switchesas used herein may also refer to sampling or sampling switches,respectively. Target response filter 54 calculates a subsampled expectedpredetection signal {circumflex over (d)}_(i), or the signal that isexpected to be output from adjustable filter 44. Particular embodimentsmay include a target response filter positioned before subsamplingoccurs, as illustrated in FIG. 5.

Subsampling switch 52 b subsamples the sampled signal {tilde over(d)}_(k) at sampling time k=iN to generate subsampled signal {tilde over(d)}_(i). At summation node 55, the subsampled signal {tilde over(d)}_(i) is subtracted from the expected subsampled signal {circumflexover (d)}_(i) to yield error e_(i). Thus, error e_(i) is the differencebetween the actual subsampled signal and the expected subsampled signal.

At multiplication node 57, the error e_(i) is multiplied by thenoiseless data vector {circumflex over (a)}_(i). The product of suchmultiplication is sent to adaptation matrix 56, which translates thisinformation regarding the error of cross-correlation vectore_(i)*{circumflex over (a)}_(i) into parameters or instructions foradjustable filter 44. Such instructions enter integrator bank 58, whichmay comprise one or a plurality of integrators. Integrator bank 58produces a compensation vector, filter coefficient vector c, whichcontrols adjustable filter 44 to adjust the compensation provided by thefilter. Thus, the calculation of error e_(i) for a subsampled signal isused to adjust subsequent signals passing through adjustable filter 44to compensate for distortion caused by channel 42.

Component boxes 43 and 45 may be implemented as digital circuits, analogcircuits or a combination of digital and analog circuits. The componentsof component box 43 operate at the full speed of channel 42, and thecomponents of component box 45 operate at 1/N speed of the channel. Inparticular embodiments, the subsampling rate N of subsampling switches52 may be approximately thirty-two or larger. Therefore, subsamplingswitches 52 a and 52 b enable signal processing to be performed at aslower rate than the rate of the signal passing through the channel. Thephysical characteristics of the channel do not typically varysignificantly. Thus, the error e_(i) does not need to be calculated atthe full speed of the channel. Accordingly, particular embodiments ofthe present invention may utilize subsampling to compensate forinterference. Utilizing subsampling to calculate the error e_(i) reducesthe negative impact of the circuitry needed to calculate the error e_(i)on the speed of continuous data sequence a_(k) passing through theadaptive equalization system. Thus, the speed of the data sequencethrough the equalizer may be faster, and the overall efficiency ofadaptive equalization system 40 may be improved.

FIG. 4 illustrates an adaptive equalization system 130 utilizing anaccumulator 140 to form vector output, in accordance with anotherembodiment of the present invention. Adaptive equalization system 130includes an adjustable filter 132, a detector 134, an error calculator136, a processor 138, an accumulator 140 and adaptation control 142.

Adjustable filter 132 receives a signal that has been transmitted over achannel, such as channel 16 of FIG. 1. Adjustable filter 132 compensatesfor distortion in the signal caused by the channel. Adaptation control142 provides control and adjustment parameters for adjustable filter132.

After the signal has passed through adjustable filter 132, it travels todetector 134 which recovers the originally transmitted data sequence.Error calculator 136 performs signal processing to calculate an errorassociated with the signal. The error is sampled at a particularinterval before entering processor 138. In particular embodiments, thesignal may be sampled before calculating the error. In such cases, theerror is calculated from the sampled signal.

After the error is sampled, it is received by processor 138. Processor138 uses sampled outputs of detector 134 to process data points forcross-correlation with the sampled error. The processing of processor138 is sent to accumulator 140. Accumulator 140 creates across-correlation vector output based on the serial operation ofprocessor 138. Accumulator 140 waits to receive multiple data pointscross-correlated with error from processor 138 to create thecross-correlation vector output. Processor 138 may utilize a low-passfilter and a pulse generator controlled by accumulator 140 in itsprocessing operations. The vector output of accumulator 140 is utilizedby adaptation control 142 to determine parameters or instructions foradjustable filter 132. Adjustable filter 132 uses such instructions toprovide compensation to subsequent signals for distortion caused by thechannel.

In the illustrated embodiment, component box 133 includes adjustablefilter 132, detector 134 and error calculator 136; component box 135includes processor 138; and component box 137 includes accumulator 140and adaptation control 142. Component boxes 133 and 135 may beimplemented as digital circuits, analog circuits or a combination ofdigital and analog circuits. Component box 137 may be implemented as adigital circuit, analog circuit, software or a combination of thepreceding.

The components of component box 133 operate at the speed of the channelthrough which the signal is received. Since the error is sampled at aparticular interval before entering component box 135, processor 138operates at a slower speed than the speed of the components of componentbox 133. Moreover, since accumulator 140 must wait to receive multiplecross-correlated data points from sampled error and detector output, thecomponents of component box 137 operate at a slower speed than the speedof the components of component box 135.

Because adaptive equalization system 130 provides compensation fromsignal processing based on vectors produced from scalar data (as opposedto vectors produced from vector data all the time as illustrated inFIGS. 2 and 3), the circuitry of the system may be less complex.Moreover, the amount of hardware may be reduced as well. For example,adaptive equalization system 130 may not need a shift register forsignal processing that would operate at the full speed of the channelthrough which the signal is received.

FIG. 5 illustrates an adaptive equalization system 60 utilizing anaccumulator 80 for producing vector output, in accordance with anotherembodiment of the present invention. Adaptive equalization system 60utilizes accumulator 80 to produce a vector output for cross-correlationwith error instead of using a shift register as used by adaptiveequalization system 40 of FIG. 3. Adaptive equalization system 60 alsoincludes sampling switch 62, adjustable filter 64, detector 66, targetresponse filter 68, summation node 70, subsampling switches 72, pulsegenerator 74, multiplication node 76, low-pass filter 78, adaptationmatrix 82 and integrator bank 84.

Adaptive equalization system 60 receives a transmitted data sequencethat has passed through channel 61. In this embodiment, sampling switch62 is located between channel 61 and adjustable filter 64. Thus, afterthe data sequence a_(k) is transmitted over channel 61, the receivedsignal y(t) is sampled at sampling switch 62 at sampling time t=kT toyield a sampled y_(k). Sampled signal y_(k) enters adjustable filter 64which compensates for distortion caused by channel 61.

The signal then enters detector 66 which recovers the original noiselessdata sequence â_(k). Target response filter 68 calculates an expectedsampled predetection signal {circumflex over (d)}_(k), or the signalthat is expected to be output from adjustable filter 64. At summationnode 70, the sampled signal {tilde over (d)}_(k) is subtracted from theexpected sampled predetection signal {circumflex over (d)}_(k) to yieldan error e_(k), which may comprise an amplitude error of the transmittedsignal. Error e_(k) is a scalar error.

The error e_(k) is sampled at subsampling switch 72 a at sampling timek=iN to yield subsampled scalar error e_(i). Subsampling switch 72 bsubsamples original noiseless data sequence â_(k) from detector 66 atsampling time k=iN−j to yield subsampled noiseless data sequenceâ_(i,j), where N represents a sub-sampling rate, i corresponds to theindex of the sub-sampled data series and j represents a sub-samplingphase corresponding to the index of the cross-correlation vectorproduced by accumulator 80. Thus, â_(i,j) refers to â_(0N−j), â_(1N−j),â_(2N−j), â_(3N−j) . . . â_(iN−j) . . . and is scalar data. Pulsegenerator 74 produces a subsampling pulse for subsampling switches 72 aand 72 b. Accumulator 80 provides phase control to control pulsegenerator 74 and the value of j. The value of index j will cycle throughcount values to produce a vector of a particular length. For example, toproduce a vector of three elements at accumulator 80, index j may cyclethrough values 0, 1 and 2. Thus, j is a delay that is the differencebetween the phases at which subsampling switches 72 a and 72 b operate.

At multiplication node 76, the subsampled error e_(i) is multiplied bythe subsampled noiseless data sequence â_(i,j) to yield e_(i)*â_(i,j),which is sent to low-pass filter 78. Low-pass filter 78 takes an averageof multiple values of e_(i)*â_(i,j) for each value of j. For example, ifj=0, low-pass filter 78 averages multiple data points e_(0N)*â_(0N−0),e_(1N)*â_(1N−0), e_(2N)*â_(2N−0), . . . e_(nN)*â_(nN−1), . . . . If j=1,low-pass filter 78 averages multiple data points e_(0N)*â_(0N−1),e_(1N)*â_(1N−1), e_(2N)*â_(2N−1), . . . e_(nN)*â_(nN−2), . . . . If j=2,low-pass filter 78 averages multiple data points e_(0N)*â_(0N−2),e_(1N)*â_(1N−2), e_(2N)*â_(2N−2), . . . e_(nN)*â_(nN−2), . . . .Low-pass filter 78 yields E[e_(i)*â_(i,j)], which is sent to accumulator80. This process is repeated for each value of j.

Accumulator 80 accumulates E[e_(i)*â_(i,j),] for each value of j andproduces a vector output E[e_(k)*{circumflex over (a)}_(k)] which issent to adaptation matrix 82. As stated above, accumulator 80 controlsj, and the values of j correspond to the length of the vector producedby accumulator 80. Such length should be long enough to observe most ofthe ISI caused by channel 61. For example, if the pre-cursor ISI, thatis ISI before the received symbol, is effectively one symbol long andthe post-cursor ISI, that is ISI after the received symbol, iseffectively four symbols long, the length of the vector produced byaccumulator 80 should be six and the value of index j should cyclethrough values −1, 0, 1, 2, 3, 4.

Adaptation matrix 82 translates the information regarding the error ofthe vector E[e_(k)*{circumflex over (a)}_(k)] into parameters orinstructions for adjustable filter 64. Such instructions pass throughintegrator bank 84 which may include a number of integratorscorresponding to the length of a compensation vector c. Integrator bank84 produces the compensation vector, filter coefficient vector c, whichcontrols adjustable filter 64 to adjust the compensation provided by thefilter.

Component boxes 65 and 67 may be implemented as digital circuits, analogcircuits or a combination of digital and analog circuits. Component box69 may be implemented as a digital circuit, analog circuit, software ora combination of the preceding.

The components of component box 65 operate at the full speed of thechannel rate, and the components of component box 67 operate at 1/Nspeed of the channel rate. The components of component box 69 operate ata much lower speed than the channel rate, because accumulator 80 mustwait to receive averages for multiple values of j before producing itsvector output. Thus, subsampling switches 72 a and 72 b enable signalprocessing to occur both in component boxes 67 and 69 at a slower ratethan the full channel rate thus improving the full channel rate.Moreover, because adaptive equalization system 60 provides compensationbased on vectors produced from scalar data (as opposed to vectorsproduced from vector data all the time as illustrated in FIGS. 2 and 3),the circuitry of the system may be less complex, and the necessaryamount of hardware for the system may be reduced.

FIG. 6 illustrates a network system 150 utilizing adaptive equalizationwith feedback control, in accordance with a particular embodiment of thepresent invention. Network system 150 includes network elements 152 and154 coupled together by a channel 156 through which a signal may becommunicated. Network element 152 includes a transmitter 158, andnetwork element 154 includes a receiver 160. Transmitter 158 includesadjustable filter 164 and feedback monitor 167. Thus, adjustable filter164 is positioned before channel 156. Adjustable filter 164 provideserror compensation to a signal transmitted through channel 156. Receiver160 includes adaptive equalization components 162 that process areceived signal to determine instructions or control parameters foradjustable filter 164.

Network system 150 includes feedback control 166 to communicate theadjustable filter instructions or control parameters determined byadaptive equalization components 162 to adjustable filter 164 usingfeedback monitor 167 so that transmitter 158 may provide pre-emphasis toa signal through adjustable filter 164 to compensate for expecteddistortions. Feedback control 166 may communicate other information totransmitter 158 as well. In particular embodiments, feedback control 166may communicate information so that transmitter 158 may providehigh-resolution control of its output. The feedback control mechanism ofnetwork system 150 may be implemented by any of a number of ways knownto one skilled in the art. For example, the control information may becommunicated back to transmitter 158 through channel 156. Thus, the useof feedback control 166 enables transmitter 158 to provide pre-emphasisand high resolutions control of its output based on error informationdetermined by receiver 160. Therefore, efficiency of network system 150may be improved.

FIG. 7 illustrates an adaptive equalization system 100 utilizing anaccumulator and feedback control, in accordance with another embodimentof the present invention. In this embodiment, an adjustable filter 102is located before a channel 104 over which a data sequence is to betransmitted. Adjustable filter 102 may be located in a transmitter whileother signal processing components may be located in a receiver thatreceives the transmitted signal after it has passed through the channel.Feedback control may be provided from the receiver to a feedback monitorof the transmitter so that error information produced by signalprocessing components of the receiver may be utilized by adjustablefilter 102 to provide compensation in the signal before transmission.Moreover, in this embodiment error calculation is performed aftersubsampling, as also described with respect to adaptive equalizationsystem 40 of FIG. 3.

Adaptive equalization system 100 also includes sampling switch 106,detector 108, target response filter 110, subsampling switches 112,pulse generator 114, summation node 116, multiplication node 118,low-pass filter 120, accumulator 122, adaptation matrix 124 andintegrator bank 126.

As stated above, adjustable filter 102 provides compensation to a datasequence a_(k) to be transmitted over channel 104. The data sequence issampled at sampling switch 106 at sampling time t=kT to yield a sampledsignal {tilde over (d)}_(k).

The signal enters detector 108 which recovers the original noiselessdata sequence â_(k). Target response filter 110 calculates an expectedsampled predetection signal {circumflex over (d)}_(k), or the signalthat is expected to be received over channel 104.

Subsampling switch 112 a subsamples the sampled signal {tilde over(d)}_(k) at sampling time k=iN to generate subsampled signal {tilde over(d)}_(k). Subsampling switch 112 b subsamples the expected sampledpredetection signal {circumflex over (d)}_(k) at sampling time k=iN togenerate expected subsampled predetection signal {circumflex over(d)}_(i). At summation node 116, the subsampled signal {tilde over(d)}_(i) is subtracted from the expected subsampled signal {circumflexover (d)}_(i) to yield scalar error e_(i). Thus, error e_(i) is thedifference between the actual subsampled signal and the expectedsubsampled signal.

Subsampling switch 112 c subsamples the output of detector 66, originalnoiseless data sequence â_(k), at sampling time k=iN−j to yieldsubsampled noiseless data sequence â_(i,j) which is scalar data. Thus, jis a delay that is the difference between the phases at whichsubsampling switches 112 b and 112 c operate. Pulse generator 114produces a subsampling pulse for subsampling switches 112 a, 112 b and112 c. The value of j corresponds to the index of the cross-correlationvector produced by accumulator 122. Accumulator 122 provides phasecontrol to control pulse generator 114 and the value of j.

At multiplication node 118, the subsampled scalar error e_(i) ismultiplied by the subsampled noiseless data sequence â_(i,j) to yielde_(i)*â_(i,j), which is sent to low-pass filter 120. Low-pass filter 120takes an average of multiple values of e_(i)*â_(i,j) for each value of jand yields E[e_(i)*â_(i,j)], which is sent to accumulator 122.

Accumulator 122 accumulates E[e_(i)*â_(i,j)] for each value of j andproduces a vector output E[e_(k)*{circumflex over (a)}_(k)] which issent to adaptation matrix 124. Adaptation matrix 124 translates theinformation regarding the error of the vector E[e_(k)*{circumflex over(a)}_(k)] into parameters or instructions for adjustable filter 102.Such instructions pass through integrator bank 126, which may include anumber of integrators corresponding to the length of a compensationvector c. Integrator bank 126 produces the compensation vector, filtercoefficient vector C, that controls adjustable filter 102 to adjust thecompensation provided by the filter. Feedback control is utilized tocommunicate the control parameters of adjustable filter 102 back to thefilter which is located in a transmitter so that the data sequence maybe compensated before transmission through channel 104.

In particular embodiments, feedback control may communicate otherinformation back to the transmitter. For example, feedback control maycommunicate intermediate information back to the transmitter. Suchintermediate information may include the output of the accumulator orthe output of the adaptation matrix. In such cases, some components suchas the adaptation matrix and/or the integrator bank may be located atthe transmitter for calculation of the control parameters for theadjustable filter. In some cases, the error e_(i) may be communicatedfrom the receiver to the transmitter using feedback control, andsubsequent operations for calculating the control parameters for theadjustable filter may be performed at the transmitter.

Component boxes 111 and 113 may be implemented as digital circuits,analog circuits or a combination of digital and analog circuits.Component box 115 may be implemented as a digital circuit, analogcircuit, software or a combination of the preceding.

The components of component box 111 operate at the full speed of thechannel rate, and the components of component box 113 operate at 1/Nspeed of the channel rate. The components of component box 115 operateat a much lower speed than the channel rate, because accumulator 122must wait to receive averages for multiple values of j before producingits vector data output. Thus, subsampling switches 112 a, 112 b and 112c enable signal processing to occur both in component boxes 113 and 115at a slower speed than the speed of the signal transmitted through thechannel thus improving the full channel speed. Moreover, becauseadaptive equalization system 100 provides compensation based on vectorsproduced from scalar data (as opposed to vectors produced from vectordata all the time as illustrated in FIGS. 2 and 3), the circuitry of thesystem may be less complex, and the necessary amount of hardware for thesystem may be reduced.

It should be understood that particular embodiments may include anadaptive equalization system similar to that illustrated in FIG. 7 butwith the adjustable filter located after the channel, for example withthe adjustable filter located within a receiver that receives the signaltransmitted through the channel. Particular embodiments may also includean adaptive equalization system utilizing an adjustable filter dividedinto two filters, one in the transmitter and another in the receiver.

FIG. 8 is a flow chart illustrating a method 200 for providing errorcompensation to a signal using feedback control, in accordance with anembodiment of the present invention. The method begins at step 202,where error adjustment is provided to a signal at a transmitter. Theerror adjustment may be provided at an adjustable filter of thetransmitter. At step 204, the signal is communicated over the channel ata channel speed.

At step 206, the signal is received from the channel at the channelspeed. At step 208, the signal is sampled at a speed less than thechannel speed to yield a sampled signal. In particular embodiments, thesignal may be sampled at a sampling switch at a speed that is 1/32 ofthe channel speed. At step 210, error associated with the signal isdetermined. The error may comprise an amplitude error or residualintersymbol interference. The error may be determined by determining adifference between a sampled noisy signal and a sampled expectednoiseless signal generated by a target response filter. In particularembodiments, the error may then be sampled for subsequent processing.

At step 212, compensation information is determined at a speed less thanthe channel speed. The compensation information is based on the errorand may be determined at an adaptation control that includes anadaptation matrix and an integrator bank. The compensation informationmay comprise a compensation vector for use by the adjustable filter inproviding adjustment to the signal distortion. Particular embodimentsmay utilize an accumulator that accumulates a plurality of data pointsof the received signal cross-correlated with the error and forms across-correlation vector from the cross-correlated data points at thecompensation speed, which may be less than the speed at which the signalis sampled since the accumulator waits on a plurality ofcross-correlated data points before forming the cross-correlationvector. The accumulator may control a pulse generator to aid in thisprocess and may receive a plurality of averages of cross-correlated datapoints for the cross-correlation vector from a low-pass filter. Thecompensation information that is determined is based on thecross-correlation vector and may be used to provide the error adjustmentat the adjustable filter.

At step 214, information is communicated back to the transmitter forproviding error adjustment. Such communication may be performed byfeedback control which is operable to communicate information from thereceiver at which the signal is received to the transmitter thattransmits the signal. The feedback information may be communicated to afeedback monitor of the transmitter for use by the adjustable filter.The communicated information may comprise the compensation informationso that the error adjustment may be provided to the signal at thetransmitter.

In particular embodiments, steps 212 and 214 may be swapped or merged.For example, in such cases the information communicated to thetransmitter may comprise the determined error or other intermediateinformation such as the output of the accumulator or the adaptationmatrix. In such cases, the compensation information for the adjustablefilter may be determined from the intermediate information at thetransmitter.

Steps may be modified, added or omitted without departing from the scopeof the invention. Additionally, steps may be performed in any suitableorder without departing from the scope of the invention.

Although the present invention has been described in detail, variouschanges and modifications may be suggested to one skilled in the art. Itis intended that the present invention encompass such changes andmodifications as falling within the scope of the appended claims.

1. A method for providing error compensation to a signal, comprising: providing error adjustment to a signal at a transmitter; communicating the signal over a channel at a channel speed; receiving the signal from the channel at the channel speed; sampling the signal at a speed less than the channel speed to yield a sampled signal; determining an error associated with the sampled signal; determining compensation information for providing the error adjustment, the compensation information based on the error, and the compensation information determined at a compensation speed, the compensation speed less than the channel speed and less than the speed at which the signal is sampled; and communicating information to the transmitter for providing error adjustment.
 2. The method of claim 1, wherein communicating information to the transmitter comprises communicating the compensation information to the transmitter.
 3. The method of claim 1, wherein: communicating information to the transmitter comprises communicating intermediate information to the transmitter; and determining compensation information comprises determining compensation information from the intermediate information.
 4. The method of claim 3, wherein the intermediate information comprises the error associated with the signal.
 5. The method of claim 1, wherein: the sampled signal comprises a sampled noisy signal; and determining an error associated with the signal comprises: determining a sampled expected noiseless signal at a target response filter; and determining the difference between the sampled noisy signal and the sampled expected noiseless signal to determine the error.
 6. The method of claim 1, wherein the speed at which the signal is sampled is about 1/32 of the channel speed.
 7. The method of claim 1, wherein determining an error associated with the signal comprises determining an amplitude error associated with the signal.
 8. The method of claim 1, further comprising: accumulating a plurality of cross-correlated data points of the received signal cross-correlated with the error; forming a cross-correlation vector from the cross-correlated data points at the compensation speed, the compensation speed less than the speed at which the signal is sampled; and wherein the compensation information is based on the cross-correlation vector.
 9. A system for providing error compensation to a signal, comprising: an adjustable filter positioned before a channel, the adjustable filter operable to provide error adjustment to a signal; a transmitter operable to communicate the signal over the channel at a channel speed; a receiver operable to receive the signal from the channel at the channel speed; a sampling switch operable to sample the signal at a speed less than the channel speed to yield a sampled signal; an error calculator operable to determine an error associated with the sampled signal; adaptation control operable to determine compensation information for providing the error adjustment, the compensation information based on the error determined by the error calculator, and the adaptation control operable to determine the compensation information at a compensation speed, the compensation speed less than the channel speed and less than the speed at which the signal is sampled; and feedback control operable to communicate information to the adjustable filter for providing error adjustment.
 10. The system of claim 9, wherein feedback control operable to communicate information to the adjustable filter comprises feedback control operable to communicate the compensation information to the adjustable filter.
 11. The system of claim 9, wherein: feedback control operable to communicate information to the adjustable filter comprises feedback control operable to communicate intermediate information to the adjustable filter; and adaptation control operable to determine compensation information comprises adaptation control operable to determine compensation information from the intermediate information.
 12. The system of claim 9, wherein the intermediate information comprises the error associated with the signal.
 13. The system of claim 9, wherein: the sampling switch is operable to sample the signal to yield a sampled noisy signal; and the error calculator is operable to: determine a sampled expected noiseless signal at a target response filter; and determine the difference between the sampled noisy signal and the sampled expected noiseless signal to determine the error.
 14. The system of claim 9, wherein the sampling switch is operable to sample the signal at a speed that is about 1/32 of the channel speed.
 15. The system of claim 9, wherein the error calculator is operable to determine an amplitude error associated with the signal.
 16. The system of claim 9, further comprising: an accumulator operable to: accumulate a plurality of cross-correlated data points of the received signal cross-correlated with the error; and form a cross-correlation vector from the cross-correlated data points at the compensation speed, the compensation speed less than the speed at which the signal is sampled by the sampling switch; and wherein the adaptation control is operable to determine compensation information for the error adjustment based on the cross-correlation vector.
 17. A logic for providing error compensation to a signal, the logic embedded in a medium and operable to: provide error adjustment to a signal at a transmitter; communicate the signal over a channel at a channel speed; receive the signal from the channel at the channel speed; sample the signal at a speed less than the channel speed to yield a sampled signal; determine an error associated with the sampled signal; determine compensation information for providing the error adjustment, the compensation information based on the error, and the compensation information determined at a compensation speed, the compensation speed less than the channel speed and less than the speed at which the signal is sampled; and communicate information to the transmitter for providing error adjustment.
 18. The logic of claim 17, logic operable to communicate information to the transmitter comprises logic operable to communicate the compensation information to the transmitter.
 19. The logic of claim 17, wherein: logic operable to communicate information to the transmitter comprises logic operable to communicate intermediate information to the transmitter; and logic operable to determine compensation information comprises logic operable to determine compensation information from the intermediate information.
 20. The logic of claim 17, wherein the intermediate information comprises the error associated with the signal.
 21. The logic of claim 17, further operable to: accumulate a plurality of cross-correlated data points of the received signal cross-correlated with the error; form a cross-correlation vector from the cross-correlated data points at the compensation speed, the compensation speed less than the speed at which the signal is sampled; and wherein the compensation information is based on the cross-correlation vector.
 22. A system for providing error compensation to a signal, comprising: means for providing error adjustment to a signal before a channel; means for communicating the signal over the channel at a channel speed; means for receiving the signal from the channel at the channel speed; means for sampling the signal at a speed less than the channel speed to yield a sampled signal; means for determining an error associated with the sampled signal; means for determining compensation information for providing the error adjustment, the compensation information based on the error, and the compensation information determined at a compensation speed, the compensation speed less than the channel speed and less than the speed at which the signal is sampled; and means for communicating information to the means for providing error adjustment.
 23. The system of claim 22, wherein means for communicating information to the means for providing error adjustment comprises means for communicating the compensation information to the means for providing error adjustment.
 24. A method for providing error compensation to a signal, comprising: providing error adjustment to a signal at an adjustable filter positioned before a channel; communicating the signal over the channel at a channel speed; receiving the signal from the channel at the channel speed; sampling the signal at a speed less than the channel speed to yield a sampled noisy signal; determining a sampled expected noiseless signal at a target response filter; determining the difference between the sampled noisy signal and the sampled expected noiseless signal to determine an amplitude error associated with the sampled signal; accumulating a plurality of cross-correlated data points of the signal cross-correlated with the amplitude error; forming a cross-correlation vector from the cross-correlated data points at a compensation speed, the compensation speed less than the speed at which the signal is sampled; determining a compensation vector for providing the error adjustment at the adjustable filter, the compensation vector based on the cross-correlation vector, and the compensation vector determined at the compensation speed; and communicating the compensation vector to the adjustable filter for providing error adjustment. 